/* {通用样式开始} */
* {
    margin: 0;
    padding: 0;
    font-family: Arial, Helvetica, sans-serif;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

/* {通用样式结束} */

/* {导航栏基础样式} */
.header {
    position: fixed;
    top: 0;
    left: 0;
    width: 100vw;
    height: 4rem;
    padding: 0 10vw;
    box-sizing: border-box;
    display: flex;
    justify-content: space-between;
    align-items: center;
    transition: 0.5s;
    z-index: 1000;
    color: #fff;
}

.header.header_bg {
    background: #fff;
    // background: #607d8b;
    color: black;
}

.header_bg {
    position: fixed;
    box-sizing: border-box;
    display: flex;
    justify-content: space-between;
    align-items: center;
    transition: 0.5s;
    z-index: 1000;
}

.nav ul {
    margin: 0;
    padding: 0;
    display: flex;
}

.nav ul li {
    list-style: none;
    margin-left: 4rem;
}

.nav ul li a {
    color: #fff;
    padding: 0 20rem;
    font-size: 1.1em;
    text-decoration: none;
    font-weight: bold;
}

.brand a {
    color: #fff;
    font-size: 1.1em;
    text-decoration: none;
    font-weight: bold;
}